THE ANATOMY OF ALCYONIUM DIGITATUM. 357 



their natural habitat, they stretch themselves out much further 

 than they do under any circumstances in the tanks of an 

 aquarium. All that can be said, then, about this measurement 

 is that, of the polyps expanded in the tanks of an aquarium, 

 the distance from the mouth to the surface of the colony does 

 not exceed 5 mm.^ Similarly it is impossible to give any 

 reliable figures as to the exact length of the tentacles, but, from 

 observations made upon numerous living polyps with a simple 

 magnifying glass, my impression is that, when fully expanded, 

 they are normally about three quarters the height of the 

 extended portion of the polyp, that is 3 mm. 



In living Alcyoniums, as they are observed in an aquarium, 

 the tentacles are more often contracted than not, so that the 

 crown has the appearance of an eight-pointed star, as it is 

 figured in pi. xxxiv,^ fig. 3, of the second edition of Johnson's 

 ' Zoophytes,' and they will remain in this contracted condition 

 for a very long time together. It is only on exceptional occa- 

 sions that the tentacles expand themselves completely, so as 

 to show clearly their characteristic pinnate Alcyonarian appear- 

 ance. 



When the colony is killed, expanded by sudden immersion 

 in Lo Biancho's No. 2 chrom-acetic solution, and then trans- 

 ferred to methylated spirit for preservation, the zooids usually 

 lose their transparency, and become opaque, excepting at their 

 lower extremities, where the body-wall usually remains trans- 

 lucent. There is a good deal of difference, however, in the 

 degree to which they lose their transparency when preserved. 

 In some young colonies the body-walls remain perfectly trans- 

 parent, only the tentacles and stomodaeum being opaque ; and 

 occasionally the same transparency may be seen in the polvps 

 of the larger and older colonies. 



In Alcyonium palmatum of the Mediterranean the ex- 

 panded zooids, when preserved, are as a rule transparent ; this 

 is due to the fact that the body-wall is much thinner than it 

 is in the British species. 



^ In the MS. notes on the anatomy of A. palmatum, by the late Professor 

 Marshall, I find 10 mm. given for this measurement. 



